The growth of networks that are capable of handling the high data-rate transfer of voice and data signals has created a demand for optical networks. While information can be transferred optically over long distances, there is also a need for interfacing the optical portions of an optical network with electrical and electro-optical components. Thus, for example, optical networks include amplifiers for strengthening optical beams, switches for routing signals, and conversions between electrical and optical signals at either end of the network. These functions are performed by devices that include optical, electro-optical, and electrical components.
As is the case with electronic components, it is advantageous to arrange optical components and electro-optical components in a chip-like configuration that facilitates the optical and electrical interconnection between the components. More specifically, the arrangement of optical components and/or optoelectronic components, e.g., optoelectronic integrated circuits (“OEICs”), in a chip-like configuration can include the optical coupling of one optical or electro-optical component, or chip, to one another chip at the semiconductor-package level, i.e., from a first chip to a second chip, and at the circuit-board level, i.e., from a chip to a substrate.
Numerous methods have been proposed for the optical interconnection of integrated circuit chips (“ICs” or “chips”). Each of these methods has problems associated with alignment of the optical beam between the components, and/or problems associated with optical-beam transmission losses. Additional problems associated with these methods include the cost of the methods, and the manufacturing difficulty associated with the methods. Other problems occur when attempts are made to scale the proposed methods in order to accommodate a large number of optical beams.
For example, optical signal communication between two optical components can be conducted by first performing an electro-optic (“EO”) conversion of an electrical signal to an optical signal using a first complementary metal oxide semiconductor (“CMOS”) chip that is coupled to a vertical cavity surface emitting laser (“VCSEL”) chip, which functions as the light source. The resulting light beam is coupled from the VCSEL chip into a photodetector (“PD”) that is housed within yet another chip. The PD performs an optoelectric (“OE”) conversion of the received optical signal, resulting in an electrical signal that is transmitted to a second CMOS chip.
Referring to the IC package I0 shown in the vertical cross-sectional view of FIG. 1, an electro-optical chip 12 is positioned over a substrate 14, and the chip is both coupled to, and spaced apart from, the substrate using a ball grid array (“BGA”) 16 (only one solder bump of the ball grid array is shown). A light emitting component within the chip is aligned with a mirror 18 that is internal to the substrate. Signals are transmitted optically between the chip and the substrate via an optical beam 20 without an intervening material, that is, the optical interconnection between the chip and the substrate is through free space (air). Since there is nothing to guide the beam between the chip and the substrate, such an optical coupling scheme is susceptible to optical losses, mostly due to component misalignment and the divergence of the light beam.
In the case of two optoelectronic integrated circuits (“OEICs”) 12 that are connected together, the optical signals are coupled directly from one chip to the other chip, without OE conversion and/or the EO conversion by a separate VCSEL chip and/or a PD. In the case where a chip 12 is coupled to a substrate 14, the substrate typically is equipped with at least one mirror 18 and a waveguide 22, which combine to facilitate the propagation of optical signals from point to point within the substrate. The transmission distance for the optical signals within the substrate can be a much longer distance, e.g., approximately 100 millimeters (“mm”), than the optical-coupling distance “X” between the chip and the substrate, e.g., approximately 100 micrometers (“μm”).
The packaging orientation of an optically coupled chip 12 and substrate 14, in a sense, is similar to the conventional flip-chip packaging of electronic chips , e.g., CMOS chips, to a substrate, which typically is accomplished using a BGA 16.
The optical interface(s) 24 between two optical devices 12 and 14 likely coexist with electrical joints 16, because all of the aforementioned components require electric inputs and outputs in addition to the optical signal inputs and outputs. The electrical joints can be common solder bumps, pull-up solder pillars, or conductive adhesive (not shown). All of these types of electrical connectors are from approximately 80 μm to approximately 1200 μm in height.
As a result of the height of the electrical joints 16 between the devices 12 and 14, the width of an optical beam 20 output from one of the components 12 increases before the optical beam is received by another component 14. For example, in the case of a VCSEL chip, the optical beam width at the output of the chip can be approximately 20 μm and the divergence angle of the optical beam can range from approximately 9° to approximately 15°. Accordingly, the greater the distance between the component 12 that outputs the beam and the component 14 that receives the beam, the wider the width of the beam when the beam is received.
For the above example, it is estimated that an optical beam 20 output from the VCSEL chip can diverge to wider than 30 μm in diameter at the point where the beam is received by the other component, when the distance between the devices, which is determined by the height X of the solder joint(s) 16 (only one shown) between the devices, is more than approximately 40 μm. As shown in FIG. 1, once the width W of the beam becomes wider than the transverse surface area of the mirror 18 included in the substrate 14, the mirror and waveguide 22 within the substrate no longer receive all of the light output from the chip 12. Accordingly, optical signal loss can occur for the system shown in FIG. 1.
Referring additionally to vertical cross-sectional view of FIG. 2, in an effort to reduce the optical losses due to the divergence of the beam 20, microlenses 26 (only one lens is shown) may be introduced into the optical interface 24 between the devices 12 and 14, e.g., the chip and the substrate, in an effort to collimate, or converge, the beam. The microlenses are useful because they can be used to self-adjust the beam's focus when a minor shift in position occurs between the two devices. However, microlenses are problematic because they are costly, they need to be meticulously aligned with the other components, and they characteristically have backreflections that result in additional optical loss.
Free-space transmission of the optical signal between two optical components 12 and 14 is common, because free-space transmission configurations are simple and economical to create. However, because there is no physical joint between the two devices, the stability and reliability of the free space optical interface 24 can be compromised when moisture and/or dust particles get trapped between the two components.
In other efforts to reduce the optical losses due to the divergence of the beam 20 and the collection of moisture and dust, an optical polymer material has been connected between the components 12 and 14, with the polymer material acting as the optical transmission path, i.e., an optical waveguide, between the components. However, the polymer material, often in the form of bumps or balls, hardens as it cures. Because the polymer material is hard after the curing process is complete, and the polymer material is connected to both devices, the interface between the devices is not reworkable.
Reworkability of the interface 24 between the chip 12 and the substrate 14 is desirable because neither of the devices has a 100% yield. In some cases, neither of the devices, nor the combination of the two devices, can be tested before they are physically coupled together, e.g., soldered together. Even with known good dies, the optical interfaces between the two devices still need to be reworkable, because assembly yield is not always 100%. Furthermore, the shape of the polymer material can change over time, e.g., due to creeping or collapse, thus, adding to the difficulties associated with the use of polymer material that is physically connected to both of the devices.
Therefore, it would be desirable to have an optical coupling interface and related method that: (1) are compatible with existing interconnect technology, (2) result in minimal optical signal loss, (3) are reliable, (4) are reworkable, (5) are relatively inexpensive, (6) do not require extensive processing, (7) prevent the accumulation of moisture and/or particles in the optical path between the components, and (8) can be scaled for devices that transmit many optical beams 20, without the need for a microlens. The present invention satisfies these needs.